The Cell
Exercise
Q.1. Define a cell?
Answer: A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes. Cells are the structural, functional, and biological units of all living beings. A cell can replicate itself independently. Hence, they are known as the building blocks of life.
Q.2. Who discovered the cell?
Answer: Robert Hooke discovered the cell in 1665.
Q. 3. Give three examples of unicellular organisms?
Answer: Three examples of unicellular organisms are: bacteria, amoeba, and yeast.
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| Unicellular Organisms |
These are all single-celled organisms that carry out all life processes within that one cell.
4. Answer the following questions:
(i) Why cells could not be observed before 17th century?
Answer: Cells could not be observed before the 17th century primarily because they are too small to be seen with the naked eye, and the necessary technology for magnification, like the microscope, had not yet been developed. The invention of the light microscope in the 17th century enabled scientists like Robert Hooke to first observe and describe cells.
(ii) Why cork could not be observed as such by Hook?
Answer: Robert Hooke could not have observed cork "as such" (i.e., without understanding its cellular structure) because he was the first to observe its cellular structure, and he was the one who coined the term "cell" to describe the tiny, box-like compartments he saw when he examined a thin slice of cork under his microscope. Before Hooke, the cellular nature of cork was not known.
(iii) Where did Hook demonstrate his observation on cork slice?
Answer: Hook demonstrated his work to scientist at Royal Society of London.
(iv) Name the outermost layer of animal cell?
Answer: The outer most layer of animal cell is called Cell membrane or Plasma membrane.
(v) Name the layer which is present outside the plasma membrane in plant cell?
Answer: The layer immediately outside the plasma membrane in a plant cell is called the cell wall.
(vi) Where are chromosomes present in a cell?
Answer: Chromosomes are found within the nucleus of a cell. Specifically, they are located inside the nuclear membrane, which separates the nucleus from the rest of the cell.
(vii) Name the cell part that has tiny holes?
Answer: The cell part that has tiny holes is, nuclear membrane.
(viii) Name the cell organelles which are found in the plant cell?
Answer: Plant cells contain several key organelles, including chloroplasts, leucoplasts, chromoplasts, a central vacuole, cell wall, and plasmodesmata, along with those found in animal cells like the nucleus, Golgi apparatus, mitochondria, endoplasmic reticulum, ribosomes, peroxisomes, and cytoskeleton.
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| Plant Cell |
(ix) Name the cells having branched structure?
Answer: Nerve cells, also known as neurons, are the primary example of cells with a branched structure.
(x) Which cell can be observed with the unaided eye?
Answer: The largest cell that can be observed with the unaided eye is Ostrich egg.
5. Mention the functions of the following:
(a) Cell membrane (b) Chromosomes
Answer: (a) Cell Membrane: The cell membrane, also known as the plasma membrane, has several crucial functions:
- It acts as a barrier separating the cell's contents from its surroundings,
- regulates the movement of substances in and out of the cell,
- provides structural support, and
- facilitates cell signaling and adhesion.
(b) Chromosomes: Chromosomes' primary function is
to carry and organize DNA, the genetic material, ensuring its accurate replication and distribution during cell division. They also play a crucial role in transmitting genetic information from one generation to the next.
6. Why are the following important to a plant cell?
(i) Cell wall (ii) Chloroplast (iii) Mitochondria (iv) Nucleus
Answer: (i) Cell wall: The cell wall is crucial for cell survival, providing structural support, maintaining shape, protecting against external damage, and regulating cell growth. It acts as a selective barrier, controlling the movement of substances into and out of the cell, and facilitating communication between cells.
(ii) Chloroplast: Chloroplasts are crucial for plant life because they are the site of photosynthesis, where light energy is converted into chemical energy in the form of sugars. They also play a role in synthesizing other essential compounds like amino acids and fatty acids.
(iii) Mitochondria: Mitochondria are crucial for cellular life, serving as the primary powerhouses that produce energy (ATP) needed for various cellular functions. They also play vital roles in regulating metabolic activity, initiating programmed cell death (apoptosis), and influencing cellular signaling.
(iv) Nucleus: The nucleus is crucial for cell function because it houses and protects the cell's DNA, the blueprint for all cellular activities. It acts as the cell's control center, regulating processes like protein synthesis, cell division, and genetic inheritance.
7. Draw an outline diagram of an animal cell. Label the different parts?
Answer:
8. Mention three differences between plant cell and animal cell?
Answer:
| Plant Cell | Animal Cell |
|---|---|
| Has a cell wall made of cellulose. | No cell wall; only a flexible cell membrane. |
| Contains chloroplasts for photosynthesis. | No chloroplasts; cannot perform photosynthesis. |
| Has a large central vacuole. | Has small, scattered vacuoles. |
9. What features are possessed by both plant cell and animal cell?
Answer: Both plant cells and animal cells share several common features because they are eukaryotic cells. Here are the features they both possess:
1. Cell membrane – controls the movement of substances in and out of the cell.
2. Cytoplasm – jelly-like substance where most cellular activities occur.
3. Nucleus – contains DNA and controls cell functions.
4. Mitochondria – produce energy through respiration (the "powerhouse" of the cell).
5. Ribosomes – synthesize proteins.
6. Endoplasmic reticulum (ER) – involved in protein and lipid synthesis (rough ER and smooth ER).
7. Golgi apparatus (Golgi body) – packages and transports proteins and lipids.
8. Lysosomes – contain enzymes to digest worn-out cell parts (more common in animal cells, but can be found in some plant cells).
9. Peroxisomes – help in breaking down fatty acids and detoxifying harmful substances.
These shared features enable both plant and animal cells to perform basic life functions like metabolism, growth, and reproduction.
10. Why are nerve cells long? Why do these cells have projections?
Answer: Nerve cells (neurons) are long because they need to transmit electrical signals over long distances within the body — from the brain to muscles or from sensory organs to the spinal cord.
They have projections called axons and dendrites:
Axons carry signals away from the cell body to other neurons or muscles.
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Dendrites receive signals from other neurons.
These projections help neurons:
1. Communicate efficiently across large areas.
2. Connect with many other cells, forming complex networks.
Their length and structure are essential for rapid and precise signal transmission.
11. Why are mitochondria known as the 'power house of the cell'?
Answer: Mitochondria are known as the "powerhouse of the cell" because they generate most of the cell’s energy. They do this through a process called cellular respiration, where they break down glucose (sugar) and oxygen to produce ATP (adenosine triphosphate) — the main energy currency of the cell.
In simple terms:
Mitochondria take in food and oxygen.
They convert it into usable energy (ATP).
This energy powers all cell functions like movement, growth, and repair.
Without mitochondria, cells wouldn’t have the energy to survive.
12. Which four basic elements constitute more than 90% of protoplasm?
Answer: The four basic elements that constitute more than 90% of protoplasm are:
1. Carbon (C)
2. Hydrogen (H)
3. Oxygen (O)
4. Nitrogen (N)
These elements are essential for forming the major organic molecules in cells, such as proteins, carbohydrates, lipids, and nucleic acids.
13. Write in brief about the variation in shape and size of cells?
Answer: Variation in Shape and Size of Cells:
Cells vary greatly in shape and size depending on their function:
Shape: Cells can be round (e.g., red blood cells), long and thin (e.g., nerve cells), flat (e.g., skin cells), or irregular (e.g., white blood cells). The shape helps them perform specific tasks — like nerve cells being long to transmit signals.
Size: Most cells are microscopic, ranging from 0.1 to 100 micrometers. However, some can be much larger, like the ostrich egg cell, which is visible to the naked eye.
This variation allows cells to specialize and efficiently carry out different functions in the body.
14. Name the different cell organelles and the functions of these organelles?
Answer:
| Cell Organelle | Function |
|---|---|
| Nucleus | Controls cell activities and contains DNA (genetic material). |
| Mitochondria | Produces energy (ATP) through cellular respiration; known as the "powerhouse" of the cell. |
| Ribosomes | Makes proteins. Can be free in the cytoplasm or attached to the endoplasmic reticulum. |
| Rough Endoplasmic Reticulum (Rough ER) | Has ribosomes; helps in protein synthesis and transport. |
| Smooth Endoplasmic Reticulum (Smooth ER) | No ribosomes; involved in lipid synthesis and detoxification. |
| Golgi Apparatus | Modifies, packages, and ships proteins and lipids to their destination. |
| Lysosomes | Contains enzymes to digest waste materials and old organelles. Known as the "suicide bags" of the cell. |
| Vacuoles | Stores water, nutrients, and waste. Plant cells have a large central vacuole. |
| Chloroplasts (Plant Cells Only) | Site of photosynthesis; contains chlorophyll to capture sunlight. |
| Cell Membrane | Controls what enters and leaves the cell; provides protection and structure. |
| Cell Wall (Plant Cells Only) | Provides support and protection; made of cellulose. |
| Cytoplasm | Jelly-like fluid where organelles are suspended; site for many metabolic reactions. |
15. What is meant by protoplasm? How does it differ from cytoplasm?
Answer: Protoplasm:
Protoplasm is the living content of a cell, which includes the cytoplasm and the nucleus. It is often called the "physical basis of life" because it contains all components necessary for life activities.
Cytoplasm:
Cytoplasm is the part of the protoplasm that lies outside the nucleus. It includes the cytosol (fluid part), organelles (except the nucleus), and various inclusions.
Difference Between Protoplasm and Cytoplasm
| Feature | Protoplasm | Cytoplasm |
|---|---|---|
| Includes | Cytoplasm + Nucleus | Only the part outside the nucleus |
| Function | Entire cell function and life processes | Supports organelles and helps in cell processes |
| Contains | Nucleus, cytoplasm, and cell organelles | Organelles (except nucleus) and cytosol |
| Scope | Complete living part of the cell | Sub-part of the protoplasm |
16. Fill in the blanks, using the words given below:
(mycoplasma, microscope, nucleus, ostrich egg, cytoplasm, lysosome, mitochondria, Robert Hook, plant, cell membrane)
(i) The _____________ is also called as suicide bags.
Answer: lysosome
(ii) The term 'cell' was given by ____________.
Answer: Robert Hook
(iii) The instrument used to see tiny objects is called a ____________.
Answer: microscope
(iv) Smallest cell is that of a _______________.
Answer: mycoplasma
(v) An __________ is a cell that can be seen without a microscope
Answer: ostrich egg
(vi) The _____________, _________________, and __________ are parts of a cell.
Answer: nucleus, cytoplasm and cell membrane
(vii) Energy is produced in _____________.
Answer: mitochondria
(viii) Cell wall is present in _________ cells.
Answer: plant
17. Write 'True' or 'False' in front of the statements given below:
| S.No | Statement | True or False |
|---|---|---|
| (i) | Most of a cell is the nucleus | [ ] |
| (ii) | Only the nucleus of a cell represents the protoplasm | [ ] |
| (iii) | Most cells are microscopic | [ ] |
| (iv) | All living organisms are made of cells | [ ] |
| (v) | Every cell has cytoplasm | [ ] |
| (vi) | All cells in a multicellular organisms can live independently | [ ] |
| (vii) | The outer most covering in an animal cell is called cell wall | [ ] |
Answer:
| S.No | Statement | True or False |
|---|---|---|
| (i) | Most of a cell is the nucleus | [False] |
| (ii) | Only the nucleus of a cell represents the protoplasm | [False] |
| (iii) | Most cells are microscopic | [True] |
| (iv) | All living organisms are made of cells | [True] |
| (v) | Every cell has cytoplasm | [True] |
| (vi) | All cells in a multicellular organisms can live independently | [False] |
| (vii) | The outer most covering in an animal cell is called cell wall | [False] |
18. Match the statements in Column A with those in Column B
| Column A | Column B |
|---|---|
| 1. Cell | a. Outermost covering in plant cells. |
| 2. Nucleus | b. Tiny structures inside cells |
| 3. Cell wall | c. Unit of living body |
| 4. Chloroplast | d. Boss of the cell |
| 5. Cytoplasm | e. Photosynthetic units |
| 6. Organelles | f. Jelly-like substance between cell membrane and nucleus |
Answer:
| Column A | Column B |
|---|---|
| 1. Cell | c. Unit of living body |
| 2. Nucleus | d. Boss of the cell |
| 3. Cell wall | a. Outermost covering in plant cell |
| 4. Chloroplast | e. Photosynthetic units |
| 5. Cytoplasm | f. Jelly-like substance between cell membrane and nucleus |
| 6. Organelles | b. Tiny structures inside the cell |
19. Label the different parts numbered 1 to 6 of the cell indicated by guidelines in the figure shown below:
1. Cell Wall – The outermost rigid layer giving shape and protection.
2. Cell Membrane – The thin layer just inside the cell wall that controls what enters and exits the cell.
3. Cytoplasm – The jelly-like substance filling the cell, where organelles are suspended.
4. Nucleus – The control center of the cell, containing genetic material.
5. Vacuole – The large central sac storing water, nutrients, and waste.
6. Chloroplast – The green organelle where photosynthesis takes place.
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